1. Field of the Invention
The present invention relates to an information recording device, such as a digital copying machine, a laser beam printer or an optical disk system, and more particularly to an information recording device of the type in which a plurality of light sources, such as a multibeam semiconductor array, are used and the surface of an object to be scanned is simultaneously scanned with a plurality of scan lines.
2. Discussion of the Related Art
In the laser beam printer, for example, a laser beam modulated by an image signal is applied to a polygon scanner revolving at a high speed. The laser beam is reflected by the scanner, and scans the surface of a recording medium, e.g., a photoreceptor, in the main scan direction, to form a latent image on the surface. The latent image is developed into a toner image. The toner image is transferred onto a sheet of recording paper.
In this type of laser beam printer, to increase a resolution of an image or to reduce the time taken for the image formation, it is necessary to increase the revolving speed of the polygon scanner. However, physical restrictions, such as weight of the polygon scanner and torque of a drive motor, hinders the increase of the revolving speed of the polygon scanner.
To solve the problem, a multibeam scan system in which the surface of the scanned object is simultaneously scanned with a plurality of beams, has been proposed and put into practice. In this scan system, as a matter of course, a plurality of beam spots must be arrayed in a manner that those spots are made satisfactorily close to each other in the direction (referred to as a subsidiary scan direction) orthogonal to the scan direction (referred to as a main scan direction) of the scan by the polygon scanner. Aggressive efforts to manufacture a plurality of semiconductor lasers close together have been made and now progress. The spot-to-spot distance of 10 .mu.m has been achieved in the semiconductor laser arrays thus far proposed (reference is made to Japanese Patent Unexamined Publication No. Hei. 2-39583 and R. L. Thornton et al., "Properties of Closely Spaced Independently Addressable Lasers Fabricated by Impurity-Induced Disordering" Appl. Phys. Lett. 56 (17), 1623-1625 (1990)).
Also, there are optical approaches to reduce the spot-to-spot distance of the laser beams, as disclosed in Japanese Patent Unexamined Publication No. Sho. 54-7328, which uses an optical fiber or an optical waveguide for the spot-to-spot distance reduction, and Japanese Patent Unexamined Publication No. Sho. 59-15218, which uses a prism or a reflecting mirror. Additionally, Japanese Patent Unexamined Publication No. Sho. 54-38130 effectively reduces the spot-to-spot distance in the subsidiary scan direction by inclining a semiconductor laser array. Japanese Patent Unexamined Publication No. Sho. 56-110960 uses the interlaced scanning method to fill the spaces each between the adjacent laser beam spots through a plural number of scans.
An example of the spot-to-spot distance reduction method based on the interlaced scanning is shown in FIG. 19. In this example, two laser beams LB1 and LB2 are used for the interlaced scanning. In FIG. 19, d.sub.x represents the diameter of a laser beam spot that is electrophotographically defined. The laser beam spot diameter is the diameter of the spot of an image that is formed by using a laser beam having a certain light intensity distribution, and will be referred to as an electrophotographic spot diameter. The spot diameter of a laser beam is generally defined as the diameter of the beam spot of which the circumference is at an intensity of light 1/e (1/e.sup.2 in power) as great as the light intensity at the center of the beam spot. The spot diameter thus defined is called an optical spot diameter and denoted as d.sub.0. In the description to follow, the imagery spot diameter of the laser beam will be based on the optical spot diameter, unless otherwise noted.
A relationship between the optical spot diameter and the electrophotographic spot diameter is graphically illustrated in FIG. 20. A ratio of the optical spot diameter d.sub.0 to the electrophotographic spot diameter d.sub.x is called a spot-diameter correction coefficient "k". The spot-diameter correction coefficient "k" is mathematically defined as EQU k=d.sub.0 /d.sub.x ( 1)
The value of the spot-diameter correction coefficient "k" depends on the electrophotographic process used. In the process of the charged-area development where toner is attached to an area exposed to light, the coefficient "k" is preferably selected to be within 1.4.ltoreq.k.ltoreq.1.6, while in the process of discharged area development where toner is attached to an area not exposed to light, it is preferably selected to be within 1.5.ltoreq.k.ltoreq.1.8.
The center-to-center distance r.sub.3 between the two spots imaged on the surface P0 to be scanned by the two laser beams LB1 and LB2 is given by r.sub.3 =3d.sub.x. The subsidiary scan progresses by 2d.sub.x every main scan. Accordingly, as shown in FIG. 19, in the first scan, the second scan line is traced by the laser beam LB2. In the second scan, the first scan line is traced by the laser beam LB1, and the fourth scan line is traced by the laser beam LB2. Thus, a gap is formed for each scan; however, in a scan, the first laser beam of the paired ones traces a first scan line, and in the next scan, the second laser beam skips over the previously traced scan line and traces a second scan line. In this way, the scan lines are successively traced in a gapless fashion.
If the already-described semiconductor laser array having the light emitting points closely arrayed at 10 .mu.m spatial intervals (Japanese Patent Unexamined Publication No. Hei. 2-39583), is operated in the interlaced scanning mode as shown in FIG. 19, the resultant multibeam laser beam printer, in principle, would be high in definition and operating speed.
However, it is very difficult to actually manufacture the semiconductor laser array having the light emitting spots arrayed at the 10 .mu.m-intervals for the multibeam laser beam printer, for the following reasons. When the adjacent light emitting spots or semiconductor laser elements are spaced by 10 .mu.m, the thermal crosstalk between the adjacent elements becomes problematic. It was confirmed that to reduce the thermal crosstalk to such a value as to be practically negligible, the oscillation threshold value of each semiconductor laser element must be reduced to approximately 10 mA. In the case of a semiconductor laser of AlGaAs, which oscillates at approximately 780 nm in the spectrum of the infrared rays, the semiconductor laser element of such a low oscillation threshold value can be manufactured by using the technique at the present stage. In the case of a semiconductor laser of A1GaInP, which oscillates at further shorter wavelength of 680 nm, the laser elements that can be manufactured are only those each oscillating at an oscillation threshold value several times as large as that of the laser element of the AlGaAs laser.
The laser beam printer employs the electrophotographic process. The electrophotography was developed, at the beginning, for the copying machine in which the photoreceptor is exposed directly to light reflected from an original document. Some of the photoreceptors specially designed for the laser beam printer are sensitive to the infrared rays of approximately 780 nm. Such photoreceptors are unsatisfactory in lifetime and reliability performances. The photoreceptors sensitive to the infrared rays are not required for the usual copying machine. If the wavelength of laser light emitted from a semiconductor laser light source could be confined within the visible spectrum, the photoreceptor may be used for both the laser beam printer and the normal copying machine. This leads to cost reduction. Also in a case where the AlGaAs semiconductor laser oscillating at approximately 780 nm is used, when a large optical power is required for a high speed printer, a large current must be fed when it is driven, if only the threshold value is reduced. Also in this case, the thermal crosstalk problem also arises.
As described above, it is desirable to manufacture a semiconductor laser array using the A1GaInP semiconductor laser oscillating at approximately 680 nm in the visible spectrum. However, it is difficult to manufacture, by the present technique, a semiconductor laser array, which consists of the laser elements arrayed at 10 .mu.m intervals, oscillates at approximately 680 nm, and has a satisfactorily suppressed thermal crosstalk. Also in a case where the semiconductor laser having a low threshold value and oscillating at approximately 780 .mu.m, is used, it is difficult to manufacture a large power semiconductor laser array, which consists of semiconductor elements spaced by 10 .mu.m and has a satisfactorily suppressed thermal crosstalk.